Manufacturers of electronic components use a variety of techniques to fabricate semiconductor devices. One technique that has many applications is known as "plasma-assisted" processing. In plasma-assisted processing, a substantially ionized gas, usually produced by a radio-frequency electromagnetic gas discharge, provides activated neutral and ionic species that chemically react to deposit or to etch material layers on semiconductor wafers in a fabrication reactor. Reactive-ion etching (RIE), an example of plasma-assisted processes, uses the directional and energetic ions in a plasma to anisotropically etch a material layer. RIE can take place in a conventional parallel-electrode plasma processing equipment or similar semiconductor device fabrication reactor.
Applications of plasma-assisted processing for semiconductor device manufacturing include RIE processing of polysilicon, aluminum, oxides, and polyimides; plasma-enhanced chemical-vapor deposition (PECVD) of dielectrics, aluminum, and other materials; low-temperature metal-organic chemical-vapor deposition (MOCVD) of metals including aluminum and copper; low-temperature dielectric chemical-vapor deposition for planarized interlevel dielectric formation; and low-temperature growth of epitaxial semiconductor layers.
In RIE, a high-energy radio-frequency (RF) power source is applied across two parallel electrodes to produce a plasma via electrical gas discharge. Conventional plasma processes such as RIE, impose a trade-off between processing rate and semiconductor device quality. To increase the RIE processing rate requires greater plasma density and/or ion flux. The plasma density and ion flux can be increased by raising the electrical RF power absorbed within the plasma medium. Increasing the RF power to the plasma medium, however, raises the plasma ion energy levels. Ions with excessive energies may damage semiconductor devices. This is because the ions can be so energetic (hundreds of electron volts) that upon impact they penetrate and cause irradiation damage to the semiconductor device surface. When this type of ion radiation damage occurs, a post-etch surface cleaning and/or annealing process is necessary to minimize the adverse effects to the semiconductor device performance. Some RIE processes may also leave undesirable chemical deposits such as fluorohydrocarbons on the semiconductor device surface. Ultimately, the manufacturer must remove these deposits from the semiconductor device in order to prevent degradation of device fabrication yield. Due to lack of plasma confinement, the conventional plasma processing techniques may introduce various contaminants (e.g., metals into the semiconductor substrate. The contaminants can be transferred by the plasma medium via its interactions with the process chamber walls and the plasma electrodes.
The combined effects of plasma-induced surface damage and contamination produce semiconductor devices with less than optimal performance characteristics and limit fabrication process yield. Thus, with conventional plasma-assisted processing techniques, increasing RF power to increase plasma density with the intent to raise the process rate can have serious detrimental effects. If a method existed, however, to increase the plasma density and ion flux without also significantly increasing ion energies, then a manufacturer may increase plasma-assisted processing rates.
Therefore, a need exists for a method and apparatus to increase plasma density near a semiconductor wafer during plasma-assisted processing without at the same time increasing ion energy levels.
As indicated before, another limitation of conventional plasma-assisted processes derives from the fact that, during these processes, plasma disperses throughout the fabrication process chamber. In so doing, it interacts with the process chamber walls. These walls contain various metals that the activated plasma species can remove, transport to a semiconductor substrate surface, and embed into the semiconductor devices. As a result, further semiconductor device performance and reliability degradation occurs.
Consequently, there is a need for a method and apparatus to prevent plasma interaction with fabrication reactor process chamber walls during plasma-assisted processing.
To remedy the above problems, manufacturers often use a special type of plasma-assisted processing known as "magnetron-plasma-enhanced" (MPE) processing. MPE processing basically entails crossing a magnetic field with an electric field in the proximity of a semiconductor substrate during plasma processing. The crossed magnetic and electric fields cause the plasma to appear as a gaseous ball enveloping the semiconductor wafer and centered therewith. As a result, the plasma ion density is greatest around the semiconductor wafer. The plasma that the semiconductor substrate sees, therefore, does not interact significantly with the process chamber walls. MPE processing also provides improved gas excitation and higher plasma density than with the conventional plasma-assisted processes. MPE processing raises the device processing rate and reduces semiconductor device degradation from plasma-induced contaminants by making the plasma medium concentrate near the semiconductor substrate. Thus, MPE processing can produce higher semiconductor device processing rates without having to increase the local plasma ion energies.
The electric field for the magnetron-plasma-enhancement can be the result of either an externally applied DC bias or, alternatively, a self-induced plasma DC bias produced on a radio frequency (RF) power source coupled to the wafer stage and the plasma medium. Coupling an RF power source to the wafer stage results in the formation of an electric field perpendicular to the wafer surface across the plasma sheath and produces the E.times.B magnetron effect (in the presence of a transverse magnetic field). Conventional chucks for RF plasma processing, however, suffer from numerous limitations.
Conventional RF chucks used for plasma processing in a semiconductor device fabrication chamber use an RF electrode to generate the plasma. These devices usually have a large thermal mass and do not possess capability to operate over a wide range of temperatures. As a result, they have associated long thermal heat-up and cool-down transient times and cause substrate temperature nonuniformities during heating and cooling. During MPE processing, temperatures within a fabrication reactor can range from -150.degree. C. to +750.degree. C. (The conventional RF plasma chucks can usually operate either in the lower temperature range (e.g., 0.degree. C. to 200.degree. C. for plasma etch processes) or in the medium temperature range (for temperatures up to 450.degree. C. for plasma deposition processes). The conventional RF plasma chuck devices are not multipurpose and are usually incompatible with external magnetron sources. Advanced anisotropic etch processes can greatly benefit from very low or cryogenic substrate temperatures (as low as -150.degree. C.) due to elimination of lateral etch (no etch undercut) and enhanced etch selectivity. Moreover, magnetron-plasma enhancement (with or without cryogenic substrate temperature) provides additional process improvements. Magnetron-plasma-enhanced (MPE) cryogenic processing may also have important applications for deposition of thin films. MPE processing at higher temperatures (100.degree. C. up to 750.degree. C.) has important applications for thin-film (e.g. metal) deposition and plasma annealing. Capabilities for rapid wafer temperature cycling and uniform wafer heating and cooling over a wide range of temperatures (-150.degree. C. to 750.degree. C.) are essential for device fabrication throughput and yield. Conventional chucks do not provide all these capabilities together. As a result, there is a need for multipurpose RF chuck having a low thermal mass for rapid semiconductor wafer heating and cooling times. There is also a need for an MPE processing RF chuck that provides uniform wafer heating and cooling during both transient and steady-state conditions, and strong magnetic field at the substrate surface using an external magnetron source.
Other limitations associated with the conventional RF chucks for MPE processing include limited operating temperature ranges and limited magnetic field transmittance values. As temperatures exceed 500.degree. C., known RF chucks overheat and suffer from component and performance degradation. Conventional RF chucks also fail at very low or cryogenic temperatures. Thus, there is a need for an MPE processing RF source that possesses extended temperature ranges of -150.degree. C. up to 750.degree. C. with negligible component or performance degradation.
Known RF chucks also suffer from a large component thickness (e.g., over two inches) that necessarily places the semiconductor substrate a distance from an external magnetron module. A thinner RF chuck would permit placing a semiconductor wafer closer to the magnetron, thus allowing either a smaller and less expensive magnetron or a greater magnetic field strength and process uniformity for an optimal MPE effect. A need exists for an RF chuck having a smaller thickness than that of conventional devices in order to minimize the distance between a semiconductor substrate and an MPE module and, as a result, enhance the MPE process uniformity and throughput.